1. Field of the Invention
The present invention relates to a switch having a housing which receives a temperature-dependent switching mechanism and which has a first housing part on whose inner base a first electrode connected to a first external terminal is arranged, and a second housing part, closing off the first housing part, that comprises a second electrode connected to a second external terminal, the switching mechanism creating, as a function of its temperature, an electrically conducting connection between the first and the second electrode.
2. Related Prior Art
A switch of this kind is known from DE 196 09 310 A1.
In the case of the known switch, the first housing part is produced from insulating material, into which the first electrode is embedded as an integral constituent by insert-molding or encapsulation. This first housing part is closed off by a second housing part in the form of a base made of electrically conductive material, the inner side of which acts as a second electrode.
The two electrodes are, so to speak, disk-shaped sheet-metal parts on which extensions which serve as external terminals of the switch are integrally configured. The base part rests on a shoulder of the first housing part, and is retained on the latter by a hot-stamped ring.
Arranged between the two electrodes, in the interior of the housing thus constituted, is an ordinary bimetallic switching mechanism whose spring disk is braced with its rim on the base part and which, below the switching temperature, presses the movable contact element carried by it against an inwardly projecting countercontact on the other electrode. Slipped over the movable contact element, in the usual way, is a bimetallic snap disk which is unstressed below its switching temperature and, when the temperature rises above its switching point, lifts the movable contact element away from the countercontact against the force of the spring disk and thus interrupts the electrical connection between the two external terminals.
The known switch described so far is extremely robust and has very small external dimensions, so that it can be used not only universally but also, in particular, in places where little installation space is available, i.e. for example in the coils of transformers or electric motors. Via the base part, this switch is very well thermally coupled to a device being monitored, so that any rise in the temperature of the device is transferred directly into the interior of the switch and there leads to a corresponding rise in the temperature of the bimetallic snap disk. Switches of this kind are connected in series between the device to be protected and a current source, so that the operating current of the device to be protected flows through the switch, which consequently shuts off that current in the event of an impermissible temperature rise.
It is often necessary, however, to monitor not only the temperature of the device to be protected but also the operating current in terms of maintaining a specific upper limit, in order to be able to shut off the device even before the temperature rise begins. The reason is that with electric motors in particular, it often happens that because of external influences the rotor comes to a stop or rotates only very slowly, which initially leads to a rise in the operating current, which in turn results in an elevation in the temperature of the device. If the elevated current flow already causes the device to shut off, the impermissible temperature rise is entirely avoided, which of course is advantageous.
This protective function of a switch having a temperature-dependent switching mechanism is called "current-dependent" switching, and is accomplished by the fact that a series resistor, through which the operating current of the device to be protected also flows, is connected in series with the switching mechanism. By way of the selection of the resistance value of this series resistor and its thermal coupling to the switch, a specific current flow through the switch and thus through the series resistor leads to the generation of a specific quantity of heat which in turn heats up the switch and thus the bimetallic snap disk in defined fashion. The resistance can thus be used to predefine an upper limit for the operating current. If the operating current exceeds that value, the heat generated in the series resistor heats the bimetallic snap disk above its switching temperature, so that the switch opens even before the device to be protected has heated up impermissibly.
A switch of this kind is known from DE 43 36 564 A1. This switch comprises first of all an encapsulated bimetallic switching mechanism which is housed in a two-part metal housing as known, for example, from DE 21 21 802 A1.
This encapsulated switch is then arranged on a ceramic support on which a thick-film resistor, which is connected via conductor paths to the conducting lower part of the encapsulated switching mechanism, is present. The other end of the resistor is connected to a solder dot onto which a first connector lead is soldered. The second connector lead is soldered onto the electrically conductive cover part of the encapsulated switching mechanism.
Although the known switch satisfactorily makes possible current-dependent switching and at the same time allows temperature monitoring, it still has a number of disadvantages.
For one, the ceramic support cannot bear mechanical loads: during transport in bulk, hairline cracks occur which can be detected upon acceptance inspection only with a microscope. Soldering the leads onto the ceramic support often causes the conductor paths to detach. These problems require greater outlay in terms of inspection and checking, which correspondingly raises the price of the product. A further disadvantage is the low compressive stability of this design, which is not suitable for incorporation into windings of transformers or electric motors.
On the other hand, these known switches are extensively used because the attachment of a resistor having a defined resistance onto a ceramic support is a well-controlled method; here, for example, thick-film resistors are used.
A further function which is desired in temperature-dependent switches is so-called self-holding, in which when the switching mechanism is open, a residual current flows through a parallel resistor and generates sufficient heat to hold the switching mechanism open. When the switching mechanism is closed, the parallel resistor is bypassed by it, so that it now performs no function. If the switching mechanism opens, however, because either the temperature of the device or the temperature of a series resistor (because of an elevated operating current) has caused the bimetallic snap disk to kick over, the parallel resistor now carries a residual current and consequently generates sufficient heat to hold the switching mechanism open. This prevents the switch from cycling repeatedly: the switch cannot close again after the protected device has cooled down, so that the device cannot once again heat up impermissibly.
In this context, the resistance values of the parallel resistor and, if applicable, the series resistor are chosen so that the series resistor has a very low ohmic value so as to influence the current flow as little as possible, while the parallel resistor has a much higher value in order greatly to restrict the strength of the residual current, i.e. to protect the device from any excessive operating current.
A self-holding function of this kind has also been implemented in the switch known from DE 43 36 564 A1, in which there is provided on the ceramic support a PTC module which is soldered at one end to the second connector lead and at the other end to conductor paths which are connected to the lower part of the encapsulated switching mechanism.
The PTC module is thus arranged electrically in parallel with the two-part encapsulated housing and thus with the temperature-dependent switching mechanism, so that when the switching mechanism is in the closed state, it is bypassed by the latter and when the switching mechanism is in the open state, it heats up.
The self-holding function is also satisfactorily implemented with the known switch, but problems resulting from production engineering can occur if the known switch is not assembled by trained personnel. For example, it is known that the thermal behaviour of PTC modules as expressed in a typical temperature curve is influenced when the PTC modules are soldered, and improper soldering can also result in mechanical damage to the PTC module.
Thus not only is production of the known switch very wage-intensive, but a corresponding rejections rate can also occur when temporary personnel are used.